A great feature.
I tested with IE8, I can tab to the video, and access/operate all controls
EXCEPT the caption button to turn on/off or select languages. When any of
the buttons has focus the = and - keys also function to increase/decrease
the size of the captions.

I know the user can increase font size with the + key (after clicking on the
media player to give it focus). However, this particular YouTube video has a
larger default font than any others I've seen, and when if I increase it to
+3 (the maximum allowed), that is larger than +3 on other captioned videos.